This invention relates to an hydraulic vehicle braking system of the general type including a primary pressure source operable to apply braking pressure to a wheel brake actuator, sensing means arranged to sense an incipient wheel skid condition of the wheel during running of the vehicle, and a modulator device influenced in response to a signal from the sensing means to modify the braking pressure in the brake actuator and thereby the braking force applied to the wheel, in order to correct the wheel skid condition.
Some known systems of this general type employ solenoid valves to modulate fluid flow from wheel-driven pumps in order to release and re-apply the brakes in an anti-skid mode. Although such systems can be generally satisfactory in a service, they have the disadvantage of being electrically and mechanically complex and can be insufficiently sensitive to cope with extremes of operation.